This information gives you details about screening tests for Down’s Syndrome (Trisomy 21), Edwards’ Syndrome (Trisomy 18) and Patau’s Syndrome (Trisomy 13). These are offered between 11-14 weeks of your pregnancy.

This information aims to help you to make an informed choice regarding your screening options. Please read the following prior to attending the First Trimester
Screening Clinic and bring your hand held pregnancy notes with you to the appointment.

Inside all the cells of our bodies are tiny structures called chromosomes. Humans have a total of 46 chromosomes – 23 pairs, including the sex chromosomes. Chromosomes are the structures that hold all of our genes. Genes are simply the instructions that tell our body how to grow
and develop, and determine things like the colour of our eyes
and hair.

In a chromosomal condition such as a Trisomy a person has three copies of a chromosome in each cell of their body, instead of the usual two. In Down’s Syndrome a person has three copies of chromosome number 21, in Edwards’ Syndrome a person has three copies of chromosome number 18 and in Patau’s Syndrome a person has three copies of chromosome number 13.

Trisomy 21, 18 and 13 are not diseases or a hereditary condition (they are not passed down through generations of a family); it is caused by chance at conception. As yet we do not know what causes the extra chromosomes to be present, they can come from the mother or father, but we do know it occurs more commonly when mothers are over the age of 35 years.

Some people think that only older women can have a baby with a Trisomy, but this is not true. Anyone can have a baby with Down’s, Edwards’ or Patau’s Syndrome; however the risk does increase with maternal age.

Down’s Syndrome occurs around 1 in every 1000.
Edwards’ Syndrome occurs 3 in every 10,000 live births.
Patau’s Syndrome occurs 2 in every 10,000 live births.

Down’s Syndrome

Babies and children with Down’s Syndrome can be affected with learning disabilities along with medical conditions such as thyroid problems, cardiac issues, hearing and vision difficulties, among others, which may require medical input. The life expectancy of people with Down’s Syndrome is 50-60 years with some people living into their 70s.

Edwards’ Syndrome

Babies with Edwards’ Syndrome can be affected with multiple anomalies, including structural heart defects, breathing and feeding difficulties, growth restrictions, cleft lip and palate, Neurological problems – severe learning disabilities, seizures, jitters and gastrointestinal problems among others.

Patau’s Syndrome

Babies with Patau’s Syndrome can also be affected with multiple anomalies which may include, congenital heart defects in 80 per cent of babies, midline facial defects, including cleft lip and palate, small eyes which are closer to the centre of the face and nasal malformation; along with neural tube defects, urogenital, gastrointestinal problems and also abnormalities of the hands and feet.

50 per cent of Edwards’ and Patau’s Syndrome babies won’t survive to term (37-40 weeks of pregnancy) and there is an even slimmer chance of the babies surviving birth. If an Edwards’ or Patau’s Syndrome baby survives birth, their life expectancy is usually a few days/weeks. One in 10-12 will survive to see their first birthday; there is a ten per cent  chance of survival after one year.

See the NHSChoices website here for more information.

There are two parts to the screening test; the first is a blood test. This measures the amount of some substances which are released by the pregnancy naturally into the mother’s blood.

These substances have passed from the baby to mother and measuring them can help to determine your chance of having a baby with Trisomy 21, 18 and 13.

The second part of the test is an ultrasound scan; in addition to dating your pregnancy the sonographer will measure the thin film of fluid under the skin at the back of the baby’s neck. This is known as the nuchal translucency (NT). Babies with a Trisomy can have an increased amount of fluid in this area.

The results are combined with your age, weight and stage of pregnancy to work out the chance of your baby having a trisomy. This is known as the combined test and your result is given as a ratio figure. If your chance is less than 1:150 then it will be classed as a high chance result and you will be offered further testing. If it is more than 1:150 then it will be classed as a low chance result and no further tests will be required.

Most screening test results will fall into the low chance category. If you fall into the low chance category it does not mean your baby won’t be affected by a Trisomy, just that the chance of it happening is low. Some babies with a Trisomy are not identified through this screening process.

Results are expected within 3-5 days. High chance results will be telephoned to you directly, low chance results will be sent to you via post.

Please note: Having a thickened nuchal translucency (NT) can
be associated with other fetal anomalies, such as heart defects.
If you have chosen not to have the screening for trisomy 21, 18
or 13 and are booked for a dating scan only, the sonographer
will inform you if the NT appears larger than expected and may
suggest they measure the thickness of the NT as a measurement
above 3.5mm can be linked to other problems for your baby.

If the result of the screening test shows that your chance of having a baby affected by a Trisomy is high (ie. less than 1:150) you will be offered the opportunity to come and talk to the Antenatal Screening Midwife and/or a doctor about your results and choices.

The link below explains all your choices that will be discussed with you at your appointment.

Your choices after a higher-chance screening result 

More information about screening for Trisomy’s

If you want to talk with someone before making your decision you can contact the antenatal screening coordinator, a midwife who specialises in screening tests in pregnancy, on 01935 384 261 or 07990 663 302.

You can get more information about screening and Down’s Syndrome from these organisations:

Antenatal Results and Choices ARC
Helpline: 0207 631 0285



Contact a Family
Helpline: 0808 808 3555

Down’s Syndrome Association
Phone: 020 8682 4001

Ref: 14-17-109
Review: 11/19